Alzheimer's disease is characterized by the extensive deposition of fibrillar forms of beta-amyloid (AB) peptides into the extracellular space, and their compaction into senile plaques. The deposition of AB is also observed in a number of animal models that overexpress the human APP gene. Importantly, in both the human disease and murine models there is a robust microglial-mediated inflammatory response. Paradoxically, despite their activated phenotype, the microglia fail to mount a phagocytic response to the deposited AB in the intact brain. Schenk and colleagues have recently reported that either active or passive immunization of mice with anti-AB antibodies results in the prevention of amyloid deposition as well as the removal of pre-existing plaques. This is the result of immunoglobulin receptor (FcR)-mediated stimulation of phago-cytosis of the deposited fibrillar amyloid plaques by microglial cells. These data clearly demonstrate that the microglial cells have the intrinsic capacity to mount an effective phagocytic response. The central question raised by these studies is why microglial cells in the intact brain fail to phagocytose the deposited amyloid despite exhibiting a robust, activated pro-inflammatory phenotype. We believe that the solution to this problem requires a detailed understanding of the nature of the cell surface receptors engaged by the AB fibril, and their linkage to the phagocytic machinery. We provide preliminary data describing the structure of a receptor complex that associates with the amyloid fibrils and mediates the pro-inflammatory activation of microglia and stimulation of phagocytosis in vitro. We believe that the analysis of the signaling pathways linking cell surface receptors to the phagocytic machinery will allow a molecular explanation for the failure of microglia to phagocytose AB deposits in the AD brain, and may identify new therapeutic approaches to AD. 1. We propose to test in animal models the participation of microglial AB receptors in mediating the pro-inflammatory activation of microgila. We will cross mice overexpressing the human APP gene with animals in which the integrin-associated protein (IAP)/CD47, the B-class scavenger receptor CD36, or complement receptor CD18 genes have been knocked out, and seek to establish whether these receptors are essential for pro-inflammatory activation of microglial cells. 2. We propose to immunize with anti-AB antibodies the doubly mutant animals, and to establish if elements of the AB receptor complex are necessary and sufficient for phagocytic removal of the plaques. Specifically, we propose to test whether APP overexpressing mice when crossed with mice null at the IAP/CD47, CD36, CD18 or Fc(gamma)R subunit loci are able to effect the phagocytic removal of amyloid plaques. 3. We propose to determine if microglia derived from IAP/CD47 and CD36 null animals can phagocytose AB fibrils in vitro. In addition, we will determine if these receptors are essential for the pro-inflammatory activation of microglia in response to AB fibrils and FcR engagement. We will also screen for additional AB -interacting receptors. 4. We propose to investigate at a molecular level the mechanisms by which AB fibrils are phagocytosed. We will determine which signaling pathways link AB fibril binding to the activation of the phagocytic machinery.